my gun has no washer in the middle its just a round bit of rubber it used to be 4 mm but when i shot it above 60psi it would blow back in 2 the back of the gun

try this put glad wrap on the pressuer guage ,bikevalve ect and fill it up with air then put it in a big tray ald fill around it with water and look for bubbles coming from the back of the gun if there is air escaping form the back of the gun the air will also be leaking out the barrel...

I'm not following you... Why would it leak out the barrel if there is a leak around the plug, so long as it is relitivly small? (compared to the equilization port) And my equilization port is over-sized... it's just simply GIGANTIC. I'v emptied my gun through it in 7 seconds! (didn't open the ball valve very well)And I assure you - my plug does NOT leak that much!

On the other hand... I'v discovered a tear around my washer. I'm going to chamfer it for my new sheet... Do you think that'd be enough? It's 2.7 mm thick.